Corrosion is generically defined as the degradation of a metallic material through its interaction with its surrounding environment. A primary classification of this degradation, which occurs on the material's surface, is heat-dry, i.e. corrosion due to hot gases and humidity due to electrolytes in solution.
One type of aqueous corrosion involves the reaction of a metal surface with an electrolyte, also known as electrochemical interaction; this reaction produces a deterioration of the metals physical and chemical properties, which in turn accelerates its aging and destruction.
A relevant issue in this kind of corrosion phenomenon is the metal/electrolyte interaction. Here, an anodic dissolution of metals and the corresponding cathodic reduction takes place, producing a redox reaction this way. What also happens is the adsorption and formation of oxide layers, which can significantly reduce the corrosion rate by the inhibition of the chemical reactions existing in the corrosion process. The electrolyte is the place in which the subsequent chemical reactions take place, and the migration and diffusion of the corrosion byproducts and species involved in the corrosion process occur.
Another important mechanism responsible for corrosion is that induced by microorganisms (MIC), which is a process where microorganisms are involved (bacteria, fungus, algae); they either initiate, facilitate or accelerate the corrosion process. In the USA, the cost of corrosion affectation represents 3.1% of the gross domestic product of the country, and 40% of the internal corrosion in oil industry pipes is attributed to microbial activity.
The states of stress and deformation of materials also play an important role in corrosion phenomena. Different types of corrosion may occur depending on the redox reactions at the metal surface. Here, two types of corrosion are classified into those with and those without mechanical stresses involved.
Pitting corrosion involves no mechanical stress. It is especially observed in austenitic stainless steels in the presence of certain anions in electrolytes (chlorides and bromides). Pitting corrosion is caused by the local destruction of the passive protective layer of the metal with the formation of a small corrosion anode (pit). For low carbon content steels, pitting corrosion is commonly caused by the presence of sulfides in the alloy, combined with the action of bromide and/or chloride anions, which produces a low pH environment in the volume around the pit, further accelerating the corrosion process. Corrosion by erosion, on the other hand, does involve mechanical stresses and is observed in pipes transporting liquids. This type of corrosion appears when the flow speed exceeds a certain limit, due to local turbulence, or when there is formation sand flowing in the fluid. In this corrosion phenomenon, the passive surface of the metal is detached by the flow, leaving the metal bare, uncovered and susceptible to corrosion by the transported liquid.
Techniques for Corrosion Measurement
Various techniques exist for the measurement/analysis of corrosion, each based on different physical, chemical and biological phenomena, and aimed to obtain specific information to describe the corrosion in the studied system. Table 1 presents a non-exhaustive classification of corrosion tests based on the information and technique used.
TABLE 1techniques and tests to measure corrosionClassificationInformation/techniqueTests based on emission ofUltrasoundsignals: acoustic, magneticAcoustic Emissionand electrical currentEddy Current/Magnetic FluxSmart pigsChemical, biochemical andpH Measurementmicrobiological analysesGas Dissolved (O2, CO2, H2S)Metallic Ions counting (Fe2+, Fe3+)Microbiological AnalysisInformation of the operationTemperatureVelocity or Flux rate changePressurepHElectrochemical techniquesPotential Measurement.Potentio-static Measurement.Potentio-dynamic Measurement.A.C. Impedance.Monitoring in the corrosionCorrosion Couponsenvironment.Biocoupons or bioelementsElectric ResistanceLinear PolarizationHydrogen penetration.Galvanic CurrentSand erosion monitoringTechniques based onRadiographyelectromagnetic signalsThermography.processing.Laser profilometry.2D Scanning electron microscopy.Transmission electron microscopy.Atomic force MicroscopyOptic Microscopy.Ellipsometry.
Tests Based on the Emission of Acoustic, Magnetic and Electric Current Signals.
These tests are based on the emission and/or reception of: acoustic signals of low and ultrasonic frequencies, electrical current and magnetic fields. By their nature they are known to be non-destructive and require devices to emit and/or capture these types of signals. The information obtained includes; detection of the existence and depth of cavities within the materials and the determination of the corrosion affected surface morphology in three dimensions, although the detection of a pit depth is limited to the depth that the signal can reflect back to a detector, thus restricting the information on the true shape of the cavity.
Chemical, Biochemical and Microbiological Analyses
These tests are based on the measurement of existent elements which allow, cause or are byproduct of corrosion in metallic surfaces. There exist both; laboratory and field quick tests. These tests mainly determine pH, dissolved gases content, presence of metallic ions, sulfate-reducing microorganisms, reducers of Fe and Mn, oxidizers of sulfur, fermenters and producers of exopolym ers.
Information of the Operation.
This information provides direct or indirect evidence of the existence of areas subject to corrosion; among them are the velocity or change in flow velocity, existing thermal gradients, pressure or pressure gradient and the acidity/alkalinity of the flow environment.
Electrochemical Techniques
These techniques are based on the electric potential difference between two or more electrodes in the metal, which is affected by the corrosion between them. In this way, through the analysis of the potential decay on a metal surface, one can produce a map of position and extent of the corrosion.
Corrosive Environment Monitoring.
Monitoring of corrosion is vital to the oil and gas industry. It permits preventive and corrective actions to be carried out, avoiding much greater potential losses. In “Corrosion in the oil industry”, Oilfield review, Shlumberger V6(2) April 1994, Brondel, E. et al describe techniques of corrosion monitoring in the petroleum industry.
Gravimetric monitoring techniques of the corrosive environment are based on the measurement of average corrosion using corrosion coupons. A coupon mass is weighed before and after exposure to the corrosive environment; in this way it is possible to obtain the percentage of material lost over a known period of time. The methodology for the preparation of the test mass samples and the subsequent evaluation of the corrosion by this technique is documented in ASTM G1-90 (2003) “Standard Practice for Preparing, Cleaning, and Evaluating Corrosion Test Specimens”.
Biocoupons or bioelements, on the other hand, permit, after being exposed to the corrosive environment in the field, the extraction and identification of biological entities that induce corrosion. This is achieved through the application of traditional microbiological techniques or molecular biological methodologies. On the other hand, it is possible to determine the characteristic morphology of the microbiological corrosion which presents itself as small pits of varying depths and can cause damage and fracturing of metallic materials. Techniques of electron microscopy are used for the analysis of this type of corrosion.
Measurement of the lineal polarization resistance. This method quantifies the polarization resistance of an electrode exposed to a corrosive environment in order to determine the corrosion electric current. By considering the linear voltage-current response of a corroded element over a small range of values, the gradient of the linear section is the polarization resistance. This resistance is inversely proportional to the corrosion current, thus allowing calculating the rate of corrosion.
With respect to this technique, it has been reported that the addition of Nitrogen enhances the resistance to pitting corrosion in austenitic steels, reducing the potential of pitting corrosion in chlorine solutions or diminishing the mass loss in immersion tests in FeCl3. Therefore the presence of nitrogen is an important indicator of these materials resistance to pitting corrosion.
Electrochemical potentials. Pitting corrosion is also studied by electrochemical potentials and optical images. It allows evaluating this kind of corrosion and galvanic behavior of highly austenitic stainless steel alloys.
Method of Eddy Currents. This measurement technique is based on changes in the impedance of a sensor coil, due to defects in the material continuity, as an electric current passes through the material. This technique is especially useful for detecting small fractures or pits. Devices using the eddy current method have been used to identify and quantify intergranular corrosion (DOS) and the sensitivity of specimens of stainless steel (AISI 316) to this type of corrosion. The categories of sensitivity are based on “fracturing severity” after a bending test. They are, in increasing order: test specimen unaffected, fissured, fractured and broken. It has been observed that the amplitude of the Eddy currents increases with DOS.
Erosion of pipes by sands flow can be quantitatively estimated through expressions that involve velocities and angles of impact, density and ductility of the materials involved.
Techniques Based on Electromagnetic Signals Processing
These techniques are based on the use of electromagnetic radiation to analyze the morphology and composition of a sample. The precision of the obtained signals differ depending on the wavelength of the radiation utilized. Visible light radiation is exploited in both, optical microcopy and ellipsometry. Monochromatic visible light in the form of laser radiation, both in fixed and mobile devices, permits scanning of the surface of a pipe or sheet of metal in a way analogous to the acoustic technique. It is possible to use these techniques to measure the thickness of a passive oxide layer or to obtain the topography of a surface. Laser confocal microscopy allows to study the microorganisms existing within a corroded area. By scanning electronic microscopy, a beam of electrons interacting with the atoms of a material allows to define with great precision the corroded surface morphology. Structural and atomic composition of a specimen can be characterized in standalone spectrometers or spectrometers associated to electron microscopes. Electromagnetic radiation in the X-ray range permits the identification of the elements and compounds that make up a specimen attacked by corrosion.
Thermographic systems make use of radiation in the infrared to detect electrical systems that have experienced corrosion; the increase in electrical resistance of the damaged connections causes an increase in the local temperature around them
Corrosion in the Oil and Gas Industry
Corrosion in the oil and gas industry is present in nearly every component of every stage, from exploration and exploitation of hydrocarbon deposits to refining and production of the oil and gas derivatives. Table 2 presents a non-exhaustive list of conditions of corrosion and their potential solutions.
TABLE 2Causes of corrosion in the oil and gas industryCause of corrosionMethods of prevention and mitigationCorrosion of surface equipmentProtection with zinc-rich paints, use ofdue to rain, condensation andinhibitors, biocides, cathodic protection.sea-breeze dispersion inCleaning pigs, spheres, moisture trapspresence of oxygen.and use of sleeves.Corrosion products and pittingin internal and external pipingsurfaces.Corrosion in the sea wave zoneOverdesign of metallic elementsthickness, use of anti-corrosive coatingsand installation of cathodic protectionsystems with sacrificial anodes.Overloads due to theModeling and inclusion of these loadingaccumulation of crustaceansstates and forces in the structural designand algae, waves pounding andto mitigate the losses due to corrosion.accidental loads of lowfrequency, tides and operationalloads, cavitation effects.Increased pitting corrosionunder induced stresses enablesthe propagation of fracturesleading to structural failure.Corrosion of drilling bars.Structural supports of self-Cathodic protection, removing agentselevating platforms, immersedcontaining sodium chromate, zincducts, distillation towerschromate and sodium nitrate to removeattacked by sulphate reducingthe H2S.bacteria (SRB).Inhibitors of bio-films, such as triazine-Water injected for secondarybased compounds are used as biocidesproduction, together withfor controlling bacteria.formation water cause corrosionin the steel pipes due to the highconcentration of salts such aschlorides and the presence ofhydrogen sulfide, the origin ofwhich is microbiological.Galvanic corrosion due to theCathodic protection.union of two or more elementscomposed of different metals.
Corrosion monitoring is very relevant for the oil and gas industry since it allows for preventive and corrective actions to avoid big financial losses. In “Corrosion in the oil industry, 1994” Brondel, E. et al, 1994 “Corrosion in the oil industry”. Oilfield Review: 6(2), the authors describe some corrosion monitoring techniques in the hydrocarbon industry.
Microscopy Based Characterization of Pitting Corrosion
These types of corrosion have been studied with various microscopy techniques: Optical microscopy helps to determine the presence or absence of corrosion films and cavities caused by pitting. In the latter case, it permits the measurement of the cavities surface areas in the material and therefore allows determining the statistical distribution of those cavities surficial diameters. In addition, it permits the identification of coalesced and/or isolated cavities and their distribution on the metal's surface, as well as to identify surface fracture patterns; it also helps to determine the existence of inclusions and defects and, by manipulating the focus, allows making a rough estimate of the depth of pits.
Raman microscopy allows the identification, by spectroscopy analysis, of the specific chemical species produced in the corroded material. It also helps to study dynamic systems, such as the phenomenon of transport and distribution of chromates to active pits and the consequent formation of corrosion products on different timescales, from milliseconds up to several days. It also allows the determination of the chemical nature of these products by comparing their spectra with other spectra from known synthetic materials.
Scanning electron microscopy (SEM) provides additional advantages over the capabilities of light microscopes for the characterization of pitting corrosion. These instruments have a far greater resolution, down to 2 nm even under low vacuum; their magnification capabilities above 1,000,000× are unrivalled; all these features combined with its large focal depth made of SEM a standard tool in metallurgical science. Supported on their peripheral spectroscopy and nanomanipulation systems, these instruments help to identify the origin of the corrosion by noting the type of corrosion, the deposits found in the attacked area and the corrosion byproducts. For example, the presence of microorganisms, preserved in Glutaraldehyde, can be studied by electronic microscopy. The use of SEM in the characterization of pitting corrosion has evolved with the advent of new tools and techniques. Table 3 depicts the evolution on the characterization of this corrosion form using scanning electron microscopes.
TABLE No. 3Evolution of the characterization of pitting corrosionusing scanning electron microscopy (SEM).SEM technical features availabilityElectron microscopy OutputField-emission SEM and secondarysurface corrosion micrographs, FIG. No. 1 (1)electronsSEM with backscattered electrons detectorsurface corrosion micrographs with phasesidentification and microstructure, FIG. No. 2(2)SEM and probe microanalysis (EPMA)Identification and concentration of elements, FIG. 3(2)SEM with secondary electronsObservation of the pitting corrosion spread, by tiltingthe stage, at the specimen edge. FIG. No. 4(3)SEM with secondary electronsObservation of the pitting corrosion morphology byphysically sectioning the specimen, FIG. 5(3)Field emission Dual-Beam SEM with ionObservation of pitting corrosion morphology byand electron beams and micro EDS.sectioning the observation area with an ion beam,FIG. No. 6(4)Dual-Beam SEM with field emission andObservation of the interface of corrosion products onelectron beamsteel in an ion-beam sectioned area, FIG. No. 7(4)Field emission Dual-Beam SEM with ionSpatial characterization of elements existing at theand electron beams and micro EDS.pitting corrosion zone, FIG. No. 8(4)Tunneling microscopy with anIn-situ electrochemical studies with topographicelectrochemical unit, under controlledcharacterization and phase identification, before andenvironmental conditions.after the corrosion attack, FIG. No. 9(5). FIG. No.9 in section A shows the surface initial conditionsand, section B shows the same surface after twocycles of the process.
Table No. 3 shows how SEM techniques for the characterization of pitting corrosion have substantially developed in the last decade, through the arrival of observation in low-vacuum, WDS spectrometry and the inclusion of ion beams in these systems. However, morphological, textural, dimensional and directional characterization of the cavities has not been adequately addressed yet.
Use of Polymers
To the knowledge of the authors, the use of polymers to characterize corrosion pitting in test specimens has not been implemented in the oil and gas industry. N. Chawla et al., in 2003, in their document “Three Dimensional (3D) Characterization and Modeling of Porosity in Powder Metallurgy (P/M) Steels” utilized a technique involving epoxy resins to study the porosity of metallic alloys. Here, they describe a procedure involving microscopy imaging of a steel specimen previously impregnated with resin; afterwards, it was sequentially polished and photographed. The outcome was a series of images, which were integrated together by reconstruction software to obtain a digital object in three dimensions. One of the limitations of this procedure is the necessity for interpolation between each image, which smoothes out the porosity texture and thus does not correctly reproduce the original porosity system; In addition, an optical microscope was utilized to obtain the images; this limits the resolution, magnification and ultimately, the images precision.
According to the state of art, little have varied the procedures for polymer injection, this is particularly true in the morphological study of the pores present in rocks as can be observed in Pittman, E. D., y Duschatko, R. W. 1970 paper: “Use of pore casts and scanning electron microscope to study pore geometry”, Journal of Sedimentary Petrology. 40(4), 1153-1157). In the case of metals, the direct application of polymeric molds has been focused on the morphological characterization of the microporosity of fabricated alloys. However, nothing has been done to determine the morphological properties of a metal exposed to corrosion on the micro and nanometric scales and to quantify their corrosion rate. The advantages of using polymeric resins to obtain molds of the cavities caused by corrosion, is that the morphology of these cavities can be studied in great detail in three dimensions; in fact, the precise shapes of the micro- and nano-cavities, which cannot be obtained and measured by the aforementioned methods can be determined using this technique. For example, in the use of ultrasound or scanning laser methods, the maximum accessible depth depends on the orientation of the cavities with respect to the sensor; this fact limits the maximum depth that those devices can reach. In addition, the cost to acquire transversal sections by ion beam cutting, in terms of time and effort, is very high compared to the benefits; the morphological and dimensional information that can be obtained with this technology is limited by restrictions similar to those discussed for Chawla's work. Additionally, the period required to obtain one image at the dozens of micrometers scale may be very long or not possible to obtain and, if not properly carried, curtaining effects during milling may be a drawback (Table 3). Other techniques have similar limitations.
Tomographic analysis by X-rays is limited to the range of tens of micrometers and its resolution is insufficient to reconstruct, with high fidelity, even the texture of the corrosion walls of a specimen attacked by corrosion (Freire-Gormaly, M., MacLean, H., Bazylak, A. 2012 “Microct investigations and pore network reconstructions of limestone and carbonate-based rocks for deep geologic carbon sequestration.” Proceedings of the 6th International Conference on Energy Sustainability Conference, ASME2013, July 23-24, San Diego, Calif., USA; Chawla, N., Williams, J. J., Deng, X., McClimon, C., 2009 “Three Dimensional (3D) Characterization and Modeling of Porosity in Powder Metallurgy (P/M) Steels.” International Journal of Powder Metallurgy. 45(2)).
Furthermore, in the case of controlled conditions such as those where corrosion coupons and biocoupons are used, a more precise, economic and rapid determination of the corrosion's advance is possible using the technique outlined in this invention, compared to the techniques based on laser and ultrasound signals.
The aforementioned technologies used for morphological and morphometric characterization of pitting corrosion with corrosion coupons and microbiological induced corrosion with biocoupons, based on various microscopy methods, known by the patent applicants are surpassed by the present invention. None of the cited references integrally relates with a procedure for three dimensional morphological characterization of micro-y nano-cavities by SEM and the quantitative determination of the effective corrosion of metallic specimens.
It is therefore an objective of the current invention to provide of a new technique for the three dimensional morphological and morphometric characterization of micro- and nano-cavities caused by pitting corrosion on corrosion coupons and biocoupons. The technique is based on acquiring a polymer mold of these cavities inside a device called “Constant-volume Injection Chamber” (CIVC). The resin employed can be polyacrylic, polystyrenic, polyvinyl o epoxy. The foregoing includes determining the shape, dimensions and distribution of the net of cavities produced by chemical and/or microbiological induced corrosion, applying SEM techniques.
An additional object of the present invention is a procedure to quantitatively determine the effective corrosion of metallic specimens, derived from their volumetric and gravimetric properties in controlled volume and temperature conditions.
The aforementioned objectives and other objectives of the present invention will be more detailed and clearly stablished in the following chapters.